Pitch angle measuring system and method for wind turbines

ABSTRACT

The present invention relates to a wind turbine and a measuring system for determining the pitch angle of at least one blade relative to a hub of a wind turbine. The measurement comprises a first angular-velocity sensor ( 130 ) measuring a first angular velocity (φ) around a first sensor axis (e 1 ), the first angular-velocity sensor ( 130 ) fixedly attached to the at least one blade ( 100 ) with a first predetermined orientation selected to produce a first projection of the main rotational axis ( 32 ) onto the first sensor axis (e 1 ), a second angular-velocity sensor ( 140 ) measuring a second angular velocity (β) around a second sensor axis (e 2 ), the second angular-velocity sensor ( 140 ) fixedly attached to the at least one blade ( 100 ) with a second predetermined orientation selected to produce a second projection of the main rotational axis ( 32 ) onto the second sensor axis (e 2 ), the first sensor axis (e 1 ) and the second sensor axis (e 2 ) being linearly independent, and a first computational unit ( 160 ) computing a computed pitch angle (θ 1c ) based on of the first angular velocity (φ) and the second angular velocity (β). 
     The invention relates also to a method for determining an azimuthal position and azimuthal rotational velocity of the at least one blade relative to the nacelle, the azimuthal position being defined by the rotational motion of the hub relative to the nacelle.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims the benefit and priority of European ApplicationNo. 14000207.2, filed Jan. 21, 2014. The entire disclosure of the aboveapplication is incorporated by reference.

TECHNICAL FIELD

The present invention relates to a wind turbine and a measuring systemfor determining at least a blade pitch angle of a blade relative to ahub of a wind turbine. The invention relates also to a method fordetermining a pitch angle of at least one blade of a wind turbine.

BACKGROUND

Wind turbines extract power from the wind to generate electrical power.The aerodynamic power in the wind is changed using one or more bladesinto rotational power that drives a generator. To maximize the energyextracted during an entire year of operation at a particular site, thegenerator's maximum power level is chosen well below the value of theaerodynamic power associated with the maximum wind velocity expected atthe site. The wind speed at which the extracted rotational power matchesthe maximum generator power is called the rated speed of the windturbine.

Since the rated speed is well below the maximum wind speed at the site,it follows that there are many time periods during which the availableaerodynamic power is greater than the generator's maximum power level.Accordingly, wind turbines are provided with a means for extracting acontrollable and selectable amount of rotational power from theavailable aerodynamic power. Most typical in the art are means thatchange the aerodynamic angle of attack of the blade, said meanscomprising rotationally attaching the blade to a turbine hub, so as toallow rotation about a pitch axis running essentially along the span ofthe blade, and a blade-pitch actuator for rotationally moving the bladeby a commanded blade pitch angle about the pitch axis, thereby changingthe orientation of the blade with respect to the hub, and with respectto the incoming wind.

The commanded blade pitch angle is computed by the turbine centralcontrol unit. To achieve acceptable operational safety, the motion ofthe blade pitch angle must be done using closed-loop control, whereinthe blade pitch angle is measured independently of the blade-pitchactuator and the measured angular value reported to the turbine centralcontrol unit, along with the rotational speed of the hub. Themeasurement of the blade pitch angle, with respect to the hub, is donevia electro-mechanical encoders driven by the blade motion. Due tooperational safety requirements, the turbine cannot be operated and mustbe shut down when the turbine central controller looses the ability totrack the blade pitch angle of any of the blades used by the windturbine.

Mechanical pitch-angle encoders suffer from several shortcomings. Onesuch shortcoming is susceptibility to mechanical failures in the driveconnecting the blade's body to the internal workings of the encoder. Asecond shortcoming is the loss of accuracy due to abrasion and wear insaid drive. A third shortcoming is the loss of accuracy when the encoderand drive are misaligned following a service technician mistakenlystepping on the unit during servicing. It is thus, desirable to have ameasuring system for measuring at least the blade pitch-angle of theblade, the measuring system being built such that is free of mechanicalfailure, wear and tear. It is furthermore desirable to have saidmeasuring system additionally measure the hub's rotational speed, andmost desirable to have said measuring system additionally measuring theblade's azimuthal angular rotation from a predetermined blade azimuthalposition.

The rotational position and velocity of the hub can be measured usingthe combination of an accelerometer and a gyroscope. The gyroscopeprovides a measure of the rotational rate, and integration in time ofthe gyroscope signal provides a measure of the rotor angular position.However, small rate errors in the measurement of the rotational rate areunavoidable in practice, so that the computed rotor angular positionincludes an ever increasing error in time due to the continuedaccumulation of the small rate errors in the integration process. Tobound this accumulation of errors, it is well known in the art ofinertial motion sensors to combine the signals of a gyroscope with thoseof an accelerometer.

EP-A 1835,293 describe a wind turbine and a method of determining atleast one rotation parameter of a wind turbine rotor, wherein anaccelerometer bounds the error of the rotor angular position when theangular position is calculated by integrating in time the signal from agyroscope.

DE-A 102007030 shows a method for indirect determining of dynamic valuesof a wind- or water turbine using any measuring sensors. Anaccelerometer is also used to measure forces present in the wind turbineand use the measured forces to detect the rotational rate of the windturbine.

The two known methods use the presence of gravitational acceleration inthe measured signal to provide a ground-fixed reference frame againstwhich the rotational rate of the hub can be determined. Thecomputational method fundamentally depends on the identification ofgravity in the accelerometer signals. The gravity signal produces asinusoidal signal that varies with the rotor azimuthal angle, hence thesignal displays a periodicity with period equal to the time the rotortakes to complete one revolution. Accordingly, at least a full period isneeded to determine the phase of the gravity sinusoidal signal, viapeak-and-through detection or equivalent method, with usable accuracy.Consequently, the computation of the rotor speed is a time-delayed, ortime averaged, quantity and not an instantaneous measurement.

What is desired is a method to measure the pitch-angle of a wind-turbineblade that is free of electro-mechanical encoders, and that,furthermore, can provide an instantaneously accurate measurement ofpitch-angle, and that, furthermore, the measurement is free drift andsimilar errors due to the accumulation in time of measurementinaccuracies or errors.

Therefore it is an objective of the invention to provide a measuringsystem for determining at least the pitch angle of at least one bladerelative to the turbine hub. It is a further objective of the presentinvention to provide a measuring system for determining the blade pitchangle that is free from the accumulation of errors due to timeintegration of a time-varying signal. It is a further objective of thepresent invention to provide a measuring system for determining therotational speed of the hub. It is a further objective of the presentinvention to provide a measuring system for determining the azimuthalangular position of the hub relative to a predetermined orientation. Itis a further objective of the present invention to provide an improvedwind turbine having a measuring system measuring at least the pitchangle of at least one blade relative to the turbine hub.

The mentioned objectives are solved by a pitch-angle measuring systemfor a wind turbine and a method of determining the pitch angle of atleast one blade relative to the turbine. Various aspects, advantages andfeatures of the invention are apparent from the dependent claims and theaccompanying drawings.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 shows a typical wind turbine with three blades, rotationalmotions of some turbine components, and components of the invention.

FIG. 2 shows components of the invention associated with one blade,including two angular-velocity sensors, and the blade at a first pitchangle.

FIG. 3 shows components of the invention associated with one blade,including two angular-velocity sensors, and the blade after rotationinto a second pitch angle.

FIG. 4. shows components of the invention associated with one blade,including three angular-velocity sensors with associated sensor axesessentially contained in one plane.

FIG. 5. shows components of the invention associated with one blade,including three angular-velocity sensors with associated sensor axesforming an essentially orthogonal basis.

FIG. 6. shows components of the invention associated with one blade,including two angular-velocity sensors and the first computational unit.

FIG. 7. shows components of the invention associated with one blade,including two angular-velocity sensors, the first computational unit,the blade-pitch actuator, and the pitch control unit.

FIG. 8. shows components of the invention associated with three blades,including angular-velocity sensors, and the computational unit forcomputing the turbine yaw-rate about the tower axis.

FIG. 9. shows components of the invention associated with three blades,including angular-velocity sensors, and the computational unit forcomputing the blade's azimuthal angle.

FIG. 10. shows a simplified diagram of components that are used togenerate a pitch difference signal for the invention.

SUMMARY

A wind turbine comprises a hub that rotates relative to a nacelle. Therotational motion of the hub drives an electrical generator located atleast partially inside the nacelle. At least one blade is rotationallyattached to the hub to allow only a rotation of the blade relative tothe hub about a pitch axis. This rotation is typically achieved with theuse of a blade bearing located at the root of the blade. Excluding smallmaterial deformations of the hub itself, the orientation of the bladeroot relative to the hub is completely and uniquely defined by a bladepitch angle that indicates the rotational movement of the blade rootabout the pitch axis from a predefined blade position. The rotationalmotion of the hub defines a main rotational axis, about which the hubrotates with a rotational rate Ω.

The inventive method of determining at least the pitch angle θ₁ of atleast one blade relative to the turbine hub comprises the steps offixedly attaching a first angular-velocity sensor to the blade formeasuring a first angular velocity φ about a first sensor axis e₁, thefixed attachment being done at a first predetermined orientation thatdefines the orientation of first sensor axis e₁ relative to the blade.

As a consequence of the fixed attachment, and of the rotor kinematics,the first angular velocity φ is functionally dependent on both therotational rate Ω and the pitch angle θ₁. One can write this dependencyas φ (Ω, θ₁). A second angular-velocity sensor is positioned in the windturbine to generate a second angular velocity β that is functionallydependent on the rotational rate Ω. The position of the second angularvelocity sensor is chosen such that the variation of the second angularvelocity β with the pitch angle θ1, including the case of zerovariation, is different from the variation of the first angular velocityφ with the pitch angle θ1. Consequently, the determination of the pitchangle θ1 is possible once the values of the first angular velocity φ andβ the second angular velocity β are known. Accordingly, the angularrates φ and β are communicated to a first computational unit thatdetermines a computed pitch angle θ1C indicative of the blade pitchangle θ1.

In the preferred embodiment the second angular velocity sensor measurethe second angular velocity β about a second sensor axis e₂ that isfixedly attached to the blade at a second predetermined position. Thissecond predetermined position is chosen such that the first sensor axise1 and the second sensor axis e₂ are linearly independent. The linearindependence of vectors e₁ and e₂ ensures that the variation of φ (Ω,θ1) and β(Ω, e₁) with θ₁ are functionally independent, thereby allowingthe pitch angle θ_(1C) to be computed once the values of the firstangular velocity φ the second angular velocity β are known. Accordingly,the angular rates φ and β are communicated to a first computational unitthat determines a computed pitch angle θ_(1C) indicative of the bladepitch angle θ₁.

In an alternative embodiment, the second angular velocity sensor measurethe second angular velocity β about a second sensor axis e₂ that isaligned with the main rotational axis of the rotor. The magnitude of βis then equal to the main rotational rate D and does not vary with pitchangle θ₁.

Consequently, the variation of the first angular velocity φ with thepitch angle θ₁ is different from the variation of the second angularvelocity β with the pitch angle θ₁, thereby allowing the pitch angleθ_(1C) to be computed once the values of the first angular velocity φthe second angular velocity β are known. Accordingly, the angular ratesφ and β are communicated to a first computational unit that determines acomputed pitch angle θ_(1C) indicative of the blade pitch angle θ₁.

DETAILED DESCRIPTION

In reference to FIG. 1, a wind turbine 1 (see FIG. 8) comprises a tower10, a nacelle 20 attached to the tower, and a hub 30 rotationallyattached to the nacelle 20. The hub is adopted to receive andstructurally support one or more blades used for transforming theaerodynamic power of the wind into mechanical power. The rotationalmotion of the hub relative to the nacelle defines a main rotational axis32. The rotational position of the hub around the rotational axis 32defines the azimuthal position ψ (FIG. 8) of the rotor. The magnitude,in radians, of the rotor azimuthal position ψ varies from zero to 2π asthe rotor performs one complete revolution. The direction of the mainrotational axis can be mathematically describes by a unit vector n. Thehub rotates around the rotational axis 32 with a main rotational rate Ω.It is well known in the art that the main rotational axis 32 and themain rotational rate Ω can be mathematically combined into the vectorΩ=Ωn. Vector notation is used herein to facilitate the description ofcertain aspects of the invention.

A first blade 100 is rotationally attached to the hub and defines apitch axis 110 (see FIG. 2) for rotation thereabout. The rotationalangular displacement of the first blade 100 around the pitch axis 110 iscommonly denoted in the art as the blade pitch angle, herein denoted θ₁.In particular, the blade pitch angle θ₁ defines the angular displacementof the first blade 100 from a predefined first blade orientationrelative to the hub 30. This predefined first blade orientation can bedefined, for example, by geometric markings on the hub and the firstblade. The rotational attachment of first blade 100 to the hub ispreferably done through a blade-bearing connecting the root of firstblade 100 to the hub 30, so that, excluding small motions due tomaterial deformations of the hub, of the blade, and of theblade-bearing, the orientation of the root of first blade 100 relativeto the hub 30 is completely and uniquely defined by a blade pitch angle.Most turbines in use today have three blades, as shown in FIG. 1, wherea second blade is shown at 200 and a third blade is shown at 300.

To keep the electrical power produced by the wind turbine within adesired range, the power extracted from the wind by each blade must becontrolled. This control is achieved by changing the blade pitch angleof each blade to affect the aerodynamic angle of attack of each blade.Thus, for the purpose of electrical power control, it is very beneficialto measure directly the blade pitch angle of each blade.

Accordingly, the invention provides a first pitch-angle measuring system170 in first blade 100 capable of providing a computed pitch angleθ_(1C) (not shown) indicative of the blade pitch angle θ₁ of first blade100. Most preferably, each blade is equipped with its own pitch-anglemeasuring system. In reference to FIG. 1, second blade 200 has a secondpitch angle measuring system 270 providing a second computed pitch angleθ_(2C) (not shown) indicative of the blade pitch angle θ₂ of secondblade 200, and third blade 300 has a third pitch-angle measuring system370 providing a third computed pitch angle θ_(3C) (not shown) indicativeof the blade pitch angle θ₃ of third blade 300. The second and thirdpitch angle measuring systems are preferably of the same design andfunction as the first pitch-angle measuring system, as described herein.

In reference to FIG. 2, the first pitch-angle measuring system 170comprises a first angular-velocity sensor 130 fixedly attached to thefirst blade 100 at a first predetermined orientation relative to thefirst blade. A gyroscope is an example of such an angular-velocitysensor. This first angular-velocity sensor 130 measures a first angularvelocity φ about a first sensor axis indicated by unit vector e₁. Thisfirst sensor axis e₁ is fixed relative to the body of the firstangular-velocity sensor 130, hence it is also fixed relative to thefirst blade 100 itself. The first predetermined orientation is chosensuch that the first sensor axis e₁ is not essentially parallel to thepitch axis 110, so that a measurable projection of the main rotationalaxis 32 onto the first sensor axis e₁ is obtained for at least somerange of the blade pitch angle θ₁ of the first blade. In vectorial form,φ=Ωe ¹

The first angular velocity φ is communicated to a first computationalunit 160 (see FIG. 8).

In a first embodiment of the invention, the pitch-angle measuring system170 comprises a second angular-velocity sensor 24 (see FIG. 1) formeasuring the angular velocity Ω about the main rotational axis n. Thefirst computational unit 160 (see FIG. 8) receives the measured angularvelocities φ and Ω and computes the computed pitch angle θ_(1C)indicative of the blade pitch angle θ₁ based from the magnitude of,together with the measured value of the main rotational rate Ω of FIG.1:θ_(1C) =c ₁ arcos(φ/Ω)+c ₂,where “c₁” and “c₂” are constants.

The main limitation of this embodiment lies in the inability of thearc-cosine function to distinguish positive pitch angles from negativepitch angles. However, since the blade pitch angle is essentiallyrestricted to the range of zero and ninety degrees in operation, thelimitation can be overcome by choosing the first predeterminedorientation so that φ is positive when the blade pitch angle takes onthe lowest value expected during operation.

In reference to FIG. 7, the pitch-angle measuring system 170 comprises asecond angular-velocity sensor 140 fixedly attached to the first blade100 at a second predetermined orientation relative to the first bladeand measures a second angular velocity β about a second sensor axisindicated by unit vector e₂. This second sensor axis e₂ is fixedrelative to the body of the second angular-velocity sensor 140, hence itis also fixed relative to the first blade 100 itself. The secondpredetermined orientation is chosen such that a measurable projection ofthe main rotational axis 32 onto the second sensor axis e₂ is obtainedfor at least some range of the blade pitch angle θ₁ of the first blade100. In vectorial form,β=Ωe ₂

Additionally, the second predetermined orientation is chosen such thatthe vectors e₁ and e₂ are linearly independent (here the meaning oflinearly independent follows the standard meaning in the field ofmathematics, namely that there do not exists scalars a and b such that ae₁+b e₂=0). Consequently, the combination of vectors e₁ and e₂ form abasis spanning a plane having plane normal m (not shown). The linearindependence of vectors e₁ and e₂ ensures that the variation of φ (Ω,θ₁) and β(Ω, θ₁) with θ₁ are functionally independent, thereby allowingthe pitch angle θ_(1C) to be computed once the values of the firstangular velocity φ the second angular velocity β are known.

The first angular velocity φ and the second angular velocity β arecommunicated to the first computational unit 160. This computationalunit computes the computed pitch angle θ_(1C) based on the numericalvalue of the first angular velocity φ relative to the numerical value ofthe second angular velocity β. In particular, since both the first andthe second angular-velocity sensors 130 and 140 respectively, arefixedly attached to the first blade 100, and since the first bladerotates with the blade pitch angle about the pitch axis 110, it followsthat the first sensor axis e₁ and the second sensor axis e₂ have anorientation in space that is functionally dependent on the blade pitchangle θ₁. This dependency is visible by comparing the orientation of e₁and e₂ in FIG. 2 with the corresponding orientations after a change inblade pitch angle, as shown in FIG. 3. It then follows that the firstangular velocity φ=Ω e₁ and the second angular velocity β=Ω e₂ are alsofunctionally dependent on the blade pitch angle θ₁. These functionalrelations are invertible, whereby the computed pitch angle θ_(1C) can becomputed based on the value of φ relative to β. For the case when e₁ ande₂ are perpendicular to each other,θ_(1C) =c ₃ arctangent(φ,β)+c ₄.

where “c₃” and “c₄” are constants.

The constant “c₄” is chosen such that the value of the computed pitchangle θ_(1C) takes on a predetermined value, such as 0, when the firstblade 100 is positioned at the predefined orientation relative to thehub. We note that in the case that e₁ and e₂ are not perpendicular toeach other, the well known application of covariant and contra variantvectors, e₁, e₂, e¹, e², where e_(j) e^(i)=δ^(i) _(j) (the Kroneckerdelta) can be used to calculate the computed pitch angle θ_(1C). Otherapproaches can be used, such as building and storing a numerical tablecorrelating known values of θ_(1C) with the associated values of φ and βfor later access and later calculating the computed pitch angle θ_(1C)via table look-up when given the values of φ and β. This approach ispreferred when the first and second sensor axis e₁ and e₂ are not known,or measurable, to sufficient accuracy, and the numerical table is builtwith the assistance of a conventional pitch-angle electromechanicalencoder 554 (see FIG. 3).

We note that in addition to the blade pitch angle θ_(1C), the mainrotational rate Ω can be computed from the numerical values of the firstangular velocity φ and the second angular velocity β. In the case thate₁ and e₂ are perpendicular 20 to each other, and the plane spanned bye₁ and e₂ has the plane-normal m (not shown) that is perpendicular tothe main rotational axis, n, thenΩ=(φ²+β²)^(1/2)

It is straight forward and well known in the art to define equivalentequations in the cases when the plane-normal m is not perpendicular tothe main rotational axis, n, and/or when e₁ and e₂ are not perpendicularto each other. In the embodiment of FIG. 7, the first and second sensoraxis e₁ and e₂ are essentially perpendicular to each other.Additionally, to minimize the effect of centripetal acceleration and ofblade bending on the measurement of angular velocity, the first andsecond angular-velocity sensors 130 and 140, respectively, arepreferably positioned near the root of the blade.

In reference to FIG. 4, a further embodiment of the invention extendsthe pitch-angle measuring system 170 by incorporating a thirdangular-velocity sensor 150 fixedly attached to the first blade 100 at athird predetermined orientation relative to the first blade, andmeasuring a third angular velocity λ (not shown) around a third sensoraxis indicated by unit vector e₃. The measurement λ is communicated tothe first computational unit 160 (see FIG. 8). When the third sensoraxis e₃ is not essentially orthogonal to the plane spanned by e₁ and e₂,the computation of the computed pitch angle θ_(1C) is made redundant byvirtue of a first additional pitch-angular value being computable fromthe relative values of λ and φ as well as a second additionalpitch-angular value being computable from the relative values of λ andβ. The first and second additional angular values provide additionalestimates for the blade pitch angle θ₁ so that inclusion of the firstand second additional values in the calculation of the computedpitch-angle θ_(1C) can be used to reduce error by employing, forexample, averaging, and for condition-monitoring of each of theangular-velocity sensors by cross-checking the agreement between thepitch angular values given by the different angular-velocity pairings.

A further level of redundancy is obtained in a further embodiment of theinvention, wherein the wind turbine 1 further comprises angular encodersto measure the blade pitch angle. Examples of such encoders areelectromechanical encoders resolving one complete revolution of theblade pitch angle into a discrete and predetermined amount of values,typically between 1024 and 16384 values.

In reference to FIG. 3, angular encoder 554 produces an encoder signalθ_(1E) indicative of the blade pitch angle of first blade 100. Theencoder signal and the computed pitch angle θ_(1C) from firstcomputational unit 160 are 25 communicated to a first comparator unit556 (see FIG. 10), which monitors the deviation, or difference, betweenthe encoder signal θ_(1E) and the computed pitch angle θ_(1C). Apitch-differential signal 558 is generated by the first comparator unit556 indicative of the difference between θ_(1E) and θ_(1C). Possiblefurther actions taken based on the value of the pitch-differentialsignal 558 include, but are not limited to, raising an alarm indicatinghardware malfunction.

Back to FIG. 1, the values of the angular velocities φ, β, λ measured,respectively, by the angular-velocity sensors 130, 140, 150 aredominated by the main rotational rate Ω. However, tilting angular rate ηof the nacelle in the tilting direction indicated by the unit vector q,caused by bending motions of the tower 10, as well as yawing angularrate γ of the nacelle in the yaw direction indicated by unit vector p,caused by changing angular position about the tower axis 34, alsocontribute to the instantaneous angular velocity of the main rotationalaxis 32,Ω=Ωn+ηUq+γpso that contributions from η and γ appear in the values of angularvelocities φ, β, and λ. The angular rates η and γ of the main rotationalaxis 32 are independent of the blade pitch angle of the first blade.Hence, when the values of the η and γ angular rates are non zero, theymust preferably be accounted for, and compensated for, by the firstcomputational unit 160 in the computation of the computed pitch-angleθ_(1C). When signals indicative of the angular rates η and γ areavailable, they are sent to the first computational unit 160. In afurther embodiment of the invention, a tilt-rate sensor 22 is used tomeasure the tilting angular rate η and a yaw-rate sensor 23 is used tomeasure the yawing angular rate γ of the main rotational axis 32.Additionally, a hub angular encoder 24 measures the first bladeazimuthal position ψ (see FIG. 8). The tilting angular rate η, theyawing angular yaw γ and the first blade azimuthal position ψ arecommunicated to the first computational unit 160, where they are used tocompensate for the η and γ angular rates in the computation of thecomputed pitch-angle θ_(1C).

In a further embodiment for a turbine having two or more blades, asshown in FIG. 4, the third angular-velocity sensor 150 is fixedlypositioned relative to first blade 100 such that the third sensor axise₃ points essentially along the pitch axis 110. For notational elegance,we rename the signal generated by third angular-velocity sensor 150 asλ₁₀₀ (not shown). The same construction is used for pitch-anglemeasuring system 270, wherein a third angular-velocity sensor 250measures the angular velocity λ₂₀₀ of second blade 200 about thatblade's pitch axis, and pitch-angle measuring system 370 has a thirdangular-velocity sensor 350 (see FIG. 8) that measures the angularvelocity λ₃₀₀ of third blade 300 about that blade's pitch axis. Atilt-and-yaw computational unit 600 (see FIG. 8) receives the threeangular velocities λ₁₀₀, λ₂₀₀, λ₃₀₀ and the first blade 100 azimuthalposition ψ (FIG. 8), and performs a multi-blade coordinatetransformation, also known as a Coleman transformation in the art,K ₀=λ₁₀₀+λ₂₀₀+λ₃₀₀k _(s)=λ₁₀₀ sin(ψ)+λ₂₀ sin(ψ−2π/3)+λ₃₀₀ sin(ψ−4π/3)k _(c)=λ₁₀₀ cos(ψ)+λ₂₀₀ cos(ψ−2π/3)+λ₃₀₀ cos(ψ−4π/3)to obtain the constant, k₀, sine, k_(s), and cosine, k_(c), componentsof the angular velocities λ₁₀₀, λ₂₀₀, λ₃₀₀. The values of the η and γangular rates are proportional to k_(s) and k_(c), and can be easilydetermined from the values of k_(s) and k_(c). The values of the η and γangular rates are then communicated to each first computational unit160, 260, 360 (see FIG. 8) for the computation of computed pitch anglesθ_(1C), θ_(2C), and θ_(3C), respectively. For the benefit ofcompactness, the first, second and third angular velocity measurementsystems can be packaged together in a single unit, for example threeMEMS gyroscopes on a single chip.

Most preferably, the orientation of the orientation of the first sensoraxis e₁, of the second sensor axis e₂, and of the third sensor axis e₃are chosen to be mutually orthogonal so that the combination of e₁, e₂,and e₃ form an orthogonal basis for three-dimensional space. Duringnormal operation, the blade pitch angle θ₁ is changed as required byturbine control needs, and this change produces a pitch angularrotational rate about the blade's pitch axis. This orthogonality,together with alignment of the third sensor axis e₃ along the pitchaxis, removes the pitch angular rotational rate from the first angularvelocity φ around the first sensor axis e₁ and from the second angularvelocity β about the second sensor axis e₂, thereby simplifying thecomputation of the computed pitch angle θ_(1C).

In a further embodiment, the nacelle angular tilt rate η is not measureddirectly, but is estimated from a measurement of the wind thrust andfrom knowledge of the tower bending stiffness, or, alternatively, froman estimation of the wind thrust via, for example, a measurement of thegenerated electrical power, and knowledge of the tower bendingstiffness. The rotor azimuthal position ψ is usually defined as theangle between the first blade 100 and the vertical (e.g. the verticalposition relative to the nacelle), and we will use this definitionherein. Accordingly, the terminology “rotor azimuthal position” and“first blade azimuthal position” are herein used interchangeably, andboth define the same angle ψ.

In the preferred embodiment, the first blade azimuthal position ψ ismeasured by an angular encoder 24 measuring the hub rotational positionrelative to a predetermined reference point on the nacelle 20. In analternative embodiment, as shown in FIG. 9, the first blade azimuthalposition ψ is computed using time integration of the main rotationalrate Ω. In particular, a pulse generator 27 generates a synchronizingpulse when the hub azimuthal position ψ obtains a predetermined value.As an example, the placement of an inductive or optical sensor on thenacelle detecting the passage of an indicator fixedly attached to thehub provides such a synchronizing pulse. For turbines that have a mainshaft attached to the hub, the indicator can alternatively be placed onthis main shaft. The synchronizing pulse and the main rotational rate Ωmeasured by at least one pitch-angle measuring system 170 arecommunicated to an azimuth computational unit 650. This unit calculatesan angle ξ (not shown) by integrating the rotational rate Ω in time,starting from the time instant at which the synchronizing pulse isreceived. The hub rotational position ψ is then determined by adding aconstant angular offset ψ₀ (not shown) to the computed angle ξ such thatthe rotational position ψ takes on a predetermined value at apredetermined rotor position,ψ=ξψ₀

For example, the value ψ₀ can be chosen such that ψ=0 occurs when firstblade 100 is in the vertical position, pointing upwards. Thesynchronizing pulse is needed to remove the accumulation of numericalerrors in the time integration process. By restarting the timeintegration of Ω at each synchronization pulse, the length of timeintegration is limited to the period of one rotation of the hub, hencethe unbounded accumulation of numerical error is prevented. Upon thereceipt of the synchronizing signal and associated termination of theintegration process for that period, the terminal value of the angle ξis compared to the value of 2π. The difference between these twonumerical values indicates the maximum error in the calculation of therotational position ψ.

Additionally, the value of ξ can be done by assuming continuity andholds for the next revolution or also can be retroactively adjusted bylinearly scaling the value ξ of over the time period between twoconsecutive synchronization pulses to yield a terminal value of ξ=2π forone hub rotation. In particular, if the computed value ξ takes on valueξ_(p) (not shown) after the time period between two consecutive pulses,then the adjusted value, which we denote ξ_(A) (not shown), is given byξ_(A)=kε, where the scaling factor k is given by k=2π/ξ_(p) and thescaling factor ensures that ξ_(A) attains a maximum value of 2π. Such anadjustment is beneficial when the rotational position ψ is used innon-real-time calculations that are performed at some time after thecompletion of the hub revolution. Furthermore, to further reduce thetime integration errors, the time integration period can be reducedbelow that of the period of one revolution by generating additionalsynchronizing pulses at predetermined angular positions of the hub.Further error reduction can be achieved by taking the integrand to be anaverage, or other numerical combination, of the main rotational rate Ωcomputed by two or more pitch-angle measuring units, such as a numericcombination of main rotational rate Ω₁₀₀ computed by angle measuringsystem 170, main rotational rate Ω₂₀₀ computed by angle measuring system270, and main rotational rate Ω₃₀₀ computed by angle measuring system370 (here, for clarity of notation, we have introduced the subscripts100, 200 and 300 to denote the associated blade).

The scaling factor k is affected by temperature, and other variablesthat change on a time period much longer than the period of a rotorrotation. Accordingly, the value of k computed at the end of a firsttime period between two consecutive synchronization pulses is usedduring the second, successive, time period to yield a corrected value ofthe rotational position,ψ=kε+ψ ₀for time values between the start and the end of the second time period.At the end of the second time period the procedure is repeated, namely anew value of k is computed at the end of the second time period and isused during the third, successive, time period. Using this procedure, animproved value of the rotational position ψ can be generatedinstantaneously for real-time algorithms or similar uses.

In reference to FIG. 7, the inventive wind turbine comprises ablade-pitch actuator 180 for changing the blade pitch angle θ₁ of thefirst blade 100, and a pitch control unit 182 in communication with theblade-pitch actuator 180. The pitch control unit 182 computes acommanded pitch angle θ_(1k) (not shown) required to maintain theextracted mechanical power driving the electrical generator within adesired range or value. This commanded pitch-angle θ_(1k) iscommunicated to the blade-pitch actuator 180, which, in turn, rotatesthe blade to the commanded pitch angle θ_(1K). The first computationalunit 160 computes the computed pitch angle θ_(1C) and communicatesθ_(1C) to the pitch control unit 182. The pitch control unit 182compares the commanded pitch angle θ_(1K) to the computed pitch angleθ_(1C) and takes corrective measures when the deviation between θ_(1K)and θ_(1C) exceeds a predetermined value.

When the turbine has multiple blades, it is known in the art thatcontrolling the pitch-angle of each blade, either collectively orindividually, achieves best control of the total mechanical powerextracted from the wind. Thus, it is most desired to apply the inventionto all blades present. The first computational unit 160, thetilt-and-yaw computational unit 600, and the azimuth computational unit650 (FIG. 9) may either be implemented as a hardware module, or as asoftware module.

LIST OF REFERENCE

-   1 wind turbine-   10 tower-   20 nacelle-   22 tilt-rate sensor-   23 yaw-rate sensor-   24 hub angular encoder-   27 pulse generator-   30 hub-   32 main rotational axis-   100 first blade-   110 pitch axis-   130 first angular velocity sensor-   140 second angular velocity sensor-   150 third angular velocity sensor-   160 first computational unit-   170 first pitch-angle measuring system-   180 blade-pitch actuator-   182 pitch control unit-   200 second blade-   270 second pitch-angle measuring system-   300 third blade-   370 third pitch-angle measuring system-   600 tilt-and-yaw computational unit-   650 azimuth computational unit-   Ω main rotational rate-   φ first angular velocity-   β second angular velocity-   λ third angular velocity-   ψ first blade azimuthal position-   η tilting angular rate-   γ yawing angular rate-   e₁ first sensor axis-   e₂ second sensor axis-   e₃ third sensor axis-   θ₁ first blade pitch angle-   θ₂ second blade pitch angle-   θ₃ third blade pitch angle-   n unit vector along the main rotational axis-   p unit vector along the tower axis-   q unit vector along the nacelle tilt axis-   m unit vector along the plane-normal of the plane spanned by e₁ and    e₂

The invention claimed is:
 1. A pitch-angle measuring system for a windturbine having a tower, a nacelle attached thereto, a hub rotationallyattached to the nacelle and defining a main rotational axis, at leastone blade rotationally attached to the hub and defining a pitch axis forrotation thereabout, a blade pitch angle defining the rotationalposition of the at least one blade with respect to the hub about saidpitch axis, the measurement system comprising: a first angular-velocitysensor measuring a first angular velocity (φ) around a first sensor axis(e₁), the first angular-velocity sensor fixedly attached to the at leastone blade with a first predetermined orientation selected to produce afirst projection of the main rotational axis onto the first sensor axis(e₁); a second angular-velocity sensor measuring a second angularvelocity (β) around a second sensor axis (e₂), the secondangular-velocity sensor fixedly attached to the at least one blade witha second predetermined orientation selected to produce a secondprojection of the main rotational axis onto the second sensor axis (e₂);the first sensor axis (e₁) and the second sensor axis (e₂) beinglinearly independent; and a first computational unit computing acomputed pitch angle (θ_(1c)) indicative of the blade pitch angle of thefirst angular velocity (φ) and the second angular velocity (β).
 2. Themeasuring system of claim 1, wherein the first computational unitcomputes the main rotational rate (Ω) based on a numerical value of thefirst angular velocity (φ) and the second angular velocity (β).
 3. Themeasuring system of claim 1 or 2, further comprising a thirdangular-velocity sensor fixedly attached to the at least one blade witha third predetermined orientation and measuring a third angular velocity(λ) around a third sensor axis (e₃), said third predeterminedorientation producing a projection of the main rotational axis onto thethird sensor axis (e₃), the third angular velocity (λ) beingcommunicated to the first computational unit, the first computationalunit including said third angular velocity (λ) value in the computationof the computed pitch angle (θ_(1C)), and said third sensor axis (e₃)being essentially aligned with the pitch axis.
 4. The measuring systemof claim 3, wherein the first sensor axis (e₁), the second sensor axis(e₂) and the third sensor axis (e₃) form essentially a three-dimensionalorthogonal basis.
 5. A pitch-angle measuring system for a wind turbinehaving a tower, a nacelle attached thereto, a hub rotationally attachedto the nacelle and defining a main rotational axis, the hub rotating ata main rotational rate (Ω), at least one blade rotationally attached tothe hub and defining a pitch axis for rotation thereabout, a blade pitchangle defining the rotational position of the at least one blade withrespect to the hub about said pitch axis, the measuring systemcomprising: a first angular-velocity sensor measuring a first angularvelocity (λ) around a first sensor axis (e₁), the first angular-velocitysensor fixedly attached to the at least one blade with a predeterminedorientation selected to produce a projection of the main rotational axisonto the first sensor axis (e₁); and a first computational unitcomputing a computed pitch angle (θ_(1C)) based on numerical values ofthe first angular velocity (φ) and the main rotational rate (Ω).
 6. Themeasuring system of claim 5, further comprising a tilt-rate sensor whichproduces a tilt-rate signal indicative of a bending of the tower, ayaw-rate sensor producing a yaw-rate signal indicative of the time rateof change of an angular position of the nacelle about a tower axis, andthe tilt-rate signal and the yaw-rate signal being received by the firstcomputational unit, the first computational unit including the yaw-ratesignal and the tilt-rate signal in the computation of the computed pitchangle (θ_(1C)).
 7. The measuring system of claim 5, further comprising apulse generator generating a synchronizing pulse when the hub attains apredetermined azimuthal angular position relative to the nacelle, thesynchronizing pulse being communicated to an azimuth computational unit,the azimuth computational unit integrating in time the main rotationalrate (Ω) to compute an azimuthal angle (ξ), the azimuth computationalunit using the azimuthal angle (ξ) and the synchronizing pulse tocompute the main rotational angle (ψ) of the at least one blade relativeto the nacelle.
 8. The measuring system of claim 5, further comprising ablade-pitch actuator for changing the blade pitch angle of the at leastone blade, and a pitch control unit in communication with theblade-pitch actuator and with the first computational unit, the pitchcontrol unit commanding the blade pitch angle to the blade-pitchactuator, the pitch control unit receiving the computed pitch angle(θ_(1C)).
 9. The measuring system of claim 5, further comprising anangular encoder producing an encoder signal indicative of the bladepitch angle, said encoder signal and the computed pitch angle (θ_(1C))being communicated to a first comparator unit, the first comparator unitgenerating a warning signal when a deviation between the encoder signaland the computed pitch angle differs by more than a predeterminedamount.
 10. A method for determining a pitch-angle of at least one bladeof a wind turbine having a tower, a nacelle attached thereto, a hubrotationally attached to the nacelle and defining a main rotationalaxis, the hub rotating at a main rotational rate (Ω), the at least oneblade rotationally attached to the hub and defining a pitch axis forrotation thereabout, a blade pitch angle defining the rotationalposition of the at least one blade with respect to the hub about saidpitch axis, the method for determining the pitch-angle comprising:measuring an angular velocity relative to a sensor axis (e₁), saidsensor axis (e₁) being fixedly attached to the blade at a predeterminedorientation, said predetermined orientation producing a projection ofthe main rotational axis onto the sensor axis (e₁); and computing acomputed pitch angle (θ_(1C)) indicative of the blade pitch angle, thecomputed pitch-angle (θ_(1C)) being based on a numerical value of themeasured angular velocity and a value of the main rotational rate (Ω).11. A method for determining a pitch-angle of at least one blade of awind turbine having a tower, a nacelle attached thereto, a hubrotationally attached to the nacelle and defining a main rotationalaxis, the at least one blade rotationally attached to the hub anddefining a pitch axis for rotation thereabout, a blade pitch angledefining a rotational position of the at least one blade with respect tothe hub about said pitch axis, the method for determining thepitch-angle comprising: measuring a first angular velocity relative to afirst sensor axis (e₁), said first sensor axis (e₁) being fixedlyattached to the blade at a first predetermined orientation, said firstpredetermined orientation producing a projection of the main rotationalaxis onto the first sensor axis (e₁); measuring a second angularvelocity relative to a second sensor axis (e₁), said second sensor axis(e₁) being fixedly attached to the blade at a second predeterminedorientation, said second predetermined orientation producing aprojection of the main rotational axis onto the second sensor axis (e1),said first sensor axis (e₁) and the second sensor axis (e₂) beinglinearly independent; and computing a computed pitch angle (e_(1C))indicative of the blade pitch angle, the computed pitch-angle (θ_(1C))being based on a numerical value of the first angular velocity and avalue of the second angular velocity.
 12. The method as claimed in claim11, wherein a first computational unit computes the main rotational rateΩ based on a numerical value of the first angular velocity (φ) and thesecond angular velocity (β).
 13. The method of claim 12, furthercomprising using a third angular-velocity sensor fixedly attached to theat least one blade with a third predetermined orientation and measuringa third angular velocity λ about a third sensor axis e3, whereby thethird predetermined orientation produces a projection of the mainrotational axis onto the third sensor axis e3, and the third angularvelocity (λ) communicates to the first computational unit, and the firstcomputational unit includes said third angular velocity (λ) value in thecomputation of the computed pitch angle (θ_(1C)).
 14. The method ofclaim 11, further comprising using a tilt-rate sensor to produce atilt-rate signal indicative of the time rate of change of the mainrotational axis orientation relative to ground due to a bending of thetower, providing a yaw-rate sensor producing a yaw-rate signalindicative of the time rate of change of the angular position of thenacelle about a tower axis, wherein the tilt-rate signal and theyaw-rate signal are received by a first computational unit and the firstcomputational unit includes the yaw-rate signal and the tilt-rate signalin the computation of the computed pitch angle (θ_(1C)).